The present invention relates to wireless communications, and more particularly to facilitating soft handoffs in a wireless communication system. Specifically, the present invention relates to the use of the common radio resources needed for soft handoff transmission in orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) communication systems.
Wireless communication systems divide areas of coverage into cells, each of which may be served by at least one predetermined base station. A mobile station will continuously monitor the signal strengths provided by the servicing base station of the current cell as well as those from adjacent cells. As the mobile station moves toward the edge of the current cell, the mobile station's signal strength is diminishing and increasing with regard to one base station and another respectively. The two base stations cooperate through the network, and determine whether the support of the communications should be switched to the adjacent base station from the current base station. The switching of control from one base station to another is referred to as a handoff.
A hard handoff is a handoff that drops the current connection with the current base station and then connects with the target base station. A soft handoff, on the contrary, allows the new connection to happen before dropping the old connection. First, the mobile station recognizes the viability of the second base station, and the network allows both the current and adjacent base stations to carry the call. As the mobile station move closer to the second base station and away from the first base station, the signal strength from the first base station will eventually drop below a predetermined threshold level. At this point, the first base station will drop the call and let the second base station continue servicing the call. In terms of call quality, soft handoffs have proven to be very reliable.
In the ever-continuing effort to increase data rates and capacity of wireless networks, communication technologies evolve and provide Multiple-input-multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) based systems representing an encouraging solution for the next generation high-speed data downlink access. A benefit of such systems is their high spectral efficiency wherein all of the allocated spectrum can be used by all base stations. The systems are generally considered to have a frequency reuse factor of one. Unfortunately, these systems generate strong co-channel interference, especially at cell borders.
In the OFDM based systems, for point-to-multipoint system such as that in the forward link, all resources of a base station are dedicated to a single mobile station at a time. The scheduler chooses the mobile station as much as possible with the best radio condition from among a set of mobile stations to send data to. If the set of mobile stations are large enough and that the channel fading of each mobile station is independent, there is almost always a mobile station in good radio condition to serve. Consequently, the base station avoids the expense of sending information to a mobile station in poor radio condition.
For the mobile station in cell edge, where it is in the boundary region between two or more sectors, even though the base station transmits to this mobile station with maximum power, the received signal is often received with very low power. As a result, this mobile station is in very poor radio condition and thus its data throughput is very low. This has several effects on system performance. The first is that if that particular mobile station requires a certain QoS, the base station must expend significant resources to serve this mobile station. The result of which is a significant decrease in total system throughput. The second is that the perceived mobile station experience for that mobile station is very poor due to the fact that the data rate that can be sustained with that link is very low. This is a significant issue because users expect to have the same user experience regardless of where they are located in the sector.
Consequently, soft handoff together with soft handoff group selection has been proposed to increase the throughput of mobile stations at the edge of a sector. However, in order to take advantage of soft handoff, common radio resources from the multiple sectors in the soft handoff group are needed to serve the mobile station. This is particularly complicated as the sub-carrier hopping pattern in an OFDM system may be different for each sector in order to randomize the intercell interference as seen by each mobile station. In other words, although the same logical resources are selected to serve the mobile station in soft handoff, with independent sub-carrier hopping among the different sectors, this will not necessary result in the same physical radio resources.
What are needed are technologies that will enable the efficient usage of common radio resources to be used for soft handoff in an OFDM system to improve the performance of mobile stations at the edge of the sector.